Field of the Invention
The present invention relates to a method for capacitive image acquisition which is suitable, in particular, for acquiring fingerprint images by means of sensors effecting capacitive measurement.
In the case of capacitive surface sensors, e.g. in the case of fingerprint sensors, the distance between the object to be measured (e.g. the surface of the finger) and the sensor is measured by a grid-shaped array of small conductor areas (pads). In the case of a fingerprint sensor, these conductor areas are very small and have a dimension of approximately 50 xcexcm to 100 xcexcm. Such fingerprint sensors effecting capacitive measurement are specified, for instance, in the overview article by Tartagni and Guerrieri: xe2x80x9cA 390 dpi Live Fingerprint Imager Based on Feedback Capacitive Sensing Schemexe2x80x9d in ISSCC97, pages 154, 155 and 402. The capacitances with respect to the measurement object are very small, so that parasitic capacitances e.g. with respect to the adjacent conductor or with respect to the support of the relevant sensor have an interfering effect on the measurement results. Sensitive amplifiers are necessary in order to be able to isolate the small measurement signals from the relatively large interference signals. The interference signals contained in the amplified signals can be suppressed either directly by measurement technology or after AD conversion by digital processing of the signal obtained. These measures are complicated and require a high degree of accuracy.
The object of the present invention is to provide a capacitive image acquisition method which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which is suitable, in particular, for acquiring fingerprints and can be implemented with little technical complexity.
With the above and other objects in view there is provided, in accordance with the invention, a method for capacitive image acquisition, which comprises:
a) dividing an area to be acquired as an image in a grid-shaped array into pixels assigned an assembly of electrical conductors comprising, for each pixel, a measuring conductor and a shielding conductor;
b) placing the area to be acquired as an image opposite the measuring conductors, so that a capacitance is present between the pixels and the measuring conductors in each case dependent on the relevant pixel;
c) at each pixel, connecting the measuring conductor and the shielding conductor to the same electrical potential and disconnecting from the potential;
d) at each pixel, discharging a charge present on one of the measuring conductor and the shielding conductor onto a respective collecting capacitor, and simultaneously compensating a potential difference between the measuring conductor and the shielding conductor; and
e) repeating steps c and d until the charges collected on the collecting capacitors have at least a value predetermined to be sufficient for a separate measurement of each collecting capacitor.
In accordance with an added feature of the invention, the potential difference between the respective measuring conductor and the respective shielding conductor is compensated identically for all pixels by placing the shielding conductors on the same predetermined potential.
In accordance with a concomitant feature of the invention, the potential difference between the respective measuring conductor and the respective shielding conductor is compensated separately for all the pixels, by always applying a same potential that is currently present on the measuring conductor to the respective shielding conductor.
The invention uses an arrangement of individual sensors effecting capacitive measurement which each comprise conductor areas, some of which are provided as measuring conductors and some are used as shielding conductors in order to shield the capacitances of the individual sensors from adjacent sensors. By means of transistors used as switches, a predetermined electrical potential is cyclically applied to all the conductors and the charge which accumulates thereon on account of the various capacitancesxe2x80x94caused by the imagexe2x80x94with respect to the measuring conductors is dissipated onto a collecting capacitor. During this operation, a connected compensation line, which has a feedback comparator in the preferred embodiment, ensures that the potential on the conductors remains at least approximately compensated, so that there is no electrical voltage across the capacitors and charging that is present can have arisen only through a further external capacitance, but not through undesirable displacement currents between the conductors.
A surface of an image which is to be acquired and engenders a locally variable capacitance relative to the conductors arranged in the grid, as is the case with the skin surface of a fingerprint, is arranged parallel to the area of the conductors during the measurement operation. Thus, the result is different charging of the individual measuring areas in accordance with the capacitance of the image present. By means of repeated charging and discharging of the capacitors of the individual sensors, the charge respectively accumulating thereon can be added on a further capacitor to the extent that these charges can be measured with little technical complexity. In a manner governed by the circuit used, the conductors, including the conductors provided as guard ring, are always at the same potential, so that no displacement currents occur between all the conductor areas present. The effect achieved in this way is that using a fundamentally known sensor arrangement for image acquisition, it is also possible to acquire images such as e.g. fingerprints which engender only very small capacitive differences.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for capacitive image acquisition, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.